Microscopic evaluation of cleaning efficiency of three different nickel-titanium rotary instruments.

INTRODUCTION
This study compared the cleaning efficiency of Mtwo, Race and Medin Nickel-Titanium (NiTi) rotary instruments.


MATERIALS AND METHODS
Sixty mandibular molar mesial roots were selected with angle curvatures between 25-35 degrees and divided into three groups; each containing 20 teeth. Canals were prepared with the rotary files and irrigated with 2.5% NaOCl solution after each instrument. Total of 5mL of normal saline was used as the final rinse; subsequently the canals were dried with paper points. The amount of debris and smear layer in three parts of the root canal walls was evaluated using SEM and the data were analyzed by using the Kruskal-Wallis test and the Mann-Whitney U test.


RESULTS
The results for remnant debris in the coronal part of root canals were similar, whereas in the middle third, Mtwo instruments achieved significantly better results compared to Race and Medin instruments. In the apical third of the root canals more debris was created by Race instruments.


CONCLUSION
Overall, Mtwo instruments had greater success in producing clean canals.


INTRODUCTION
Cleaning and shaping of the root canals creates a smear layer on the canal walls (1). The smear layer might constitute a source of nutrients for bacterial growth (2). It may also interfere with the action of irrigants (3) and adhesion and penetration of root canal sealers (4). When smear layer is not eliminated during root canal treatment, it may disintegrate or be removed by bacterial byproducts allowing leakage (5). Whether smear layer needs to be removed or retained remains controversial; however, there is growing evidence to support removal of the smear layer before obturation (6,7). Based on the report of the European Society of Endodontology the main objectives of root canal instrumentation are maintenance of the original canal curvature, elimination of residual pulp tissues and removal of debris (8). The rotary Nickel-Titanium (NiTi) instruments were developed to serve these purposes and gained rapid acceptance due to advantages of extreme flexibility and increased speed of canal preparation. According to some reports, rotary files with a flute design may offer some advantages in their cleaning ability. For example, sharp cutting edges seem to be superior to edges that have radial lands in cleaning the root canal (9,10

MATERIALS AND METHODS
Sixty extracted human mandibular molars were selected and the degree of mesial root curvature was determined using a computerized image processing system (Schneider technique) (12). The teeth with the angle of curvature of 25 to 35 degrees were selected and stored in 5.25% NaOCl for 1 hour to be cleaned. Coronal access was achieved using diamond burs. Only teeth with apical root canal width approximately compatible with size 10 K-file were included. A coronal reference point 14 mm from apical foramen was created by shortening all molar crowns accordingly. The teeth were randomly divided into three experimental groups of 20 teeth each. The mesial canals were instrumented with Mtwo (VDW, Munich, Germany), Race (FKG Dentaire, La-Chaux-de-fonds,Switzerland) and Medin (MEDIN,a.s. ,Czech Republic) using crown down technique. RcPrep (premier products co, USA) was used as a lubricant during instrumentation and the canals were irrigated with 2.5% NaOCl solution after each instrument. 5mL of normal saline (Samen Industries, Mashad, Iran) was used as final rinse after instrumentation and then the canals were dried.

Evaluations
Roots were split longitudinally with a diamond disk (D&Z, Diamant 74, Germany) and prepared for evaluation with a scanning electron microscope (SEM;Vega II XMU, Tescan, Czech Republic). Separate evaluations were recorded for smear layer and remaining debris in three areas of the root canals by means a numerical evaluation scale (13). The following scale was used.  Scores 1 and 2 of debris and smear layer dispersion were selected as suitable levels of canal cleanliness (14). Debris and smear layer was rated under a ×500 and ×1500 magnification respectively. The data were statistically analyzed by using the Kruskal-Wallis test and the Mann-Whitney U test, and the significance was set at P= 0.05.

RESULTS
None of instruments used for canal preparation was able to produce a completely clean canal. The scores of debris and smear layer are presented in table 1 and table 2. In the coronal part of the canals, no statistically significant differences were apparent in the remaining debris between experimental groups and 45-50% of specimens having score 1 ( Figure 1A). Mtwo files remained significantly less debris compared with Medin and Race files in the middle third of the canal walls (P<0.05). 50-70% of specimens in Medin and Race files groups showed score 3 ( Figure 1B). In the apical regions, the use of Race files resulted in significantly more residual debris compared with others (P<0.05). 40-50% of specimens in Race files group resulted in score 4 and 5 ( Figure 1C). In terms of smear layer, no significantly differences were apparent between groups (P>0.05) ( Table 2). In general the Mtwo files resulted in 28.4%, the Medin instruments in 26.7% and the Race system in 30% of specimens having scores 1 and 2. 6.7%, 10% and 13.3% of specimens in Mtwo, Medin and Race files groups resulted in score 5 respectively ( Figure 1D).

DISCUSSION
One of the most important objectives during root canal preparation is to eliminate microorganisms as much as possible from the root canal system by removing vital and/or necrotic pulp tissue, infected dentin, and debris (15). The smear layer is a superficial film of dentin particles, and vital or necrotic pulp remnants that are produced when a canal is instrumented (16). It is considered to be desirable to remove this layer due to its potential deleterious effects (7,17). Debris was defined as dentin chips, and vital or necrotic pulp remnants loosely attached to the canal walls (13). There is no doubt that debris removal is a critical issue for elimination of the microorganisms from the root canal system. The ability to achieve these aims was examined in this experimental study using SEM. SEM has been used in several studies to evaluate the cleaning efficiency of various instrumentation techniques (11,18,19). The present results indicated that partially uninstrumented areas with remaining debris are found in all experimental groups and canal regions that are in agreement with other studies (11,13). These findings emphasize the importance of irrigants for effective disinfection of the root canal system (20,21,22). In order to remove debris and smear layer the use of antibacterial solutions such as sodium

IEJ -Volume 5, Number 4, Fall 2010
hypochlorite is recommended in combination with chelating agents (17,23). Considering the objective of the present study (to evaluate the cleaning effectiveness of the different rotary files) 2.5% NaOCl alone was used as irrigant to prevent interference of other factors in removing debris and smear layer. In this study the use of Mtwo files resulted in significantly less remaining debris in the middle third of the canals (Table 1). No significant differences were found between groups in terms of smear layer removal. These results confirm the findings reported by Schafer et al. that found Mtwo instruments remained less debris compared with K3 or Race instruments without significant differences regarding smear layer removal (11). The authors mentioned that the design of Mtwo files (two sharp cutting edges and a small core diameter) may enhance its debris removal capacity.
The present results indicate that the apical thirds of the canals were less clean than the middle and coronal thirds in all groups that is inconsistent with others (20,21). Moreover, the use of Race files resulted in significantly more residual debris in the apical third of the canals. Schafer et al. also reported that Race files resulted in more debris compared with Mtwo files (11). This finding may be important because the effectiveness of irrigants is also reduced as we get closer to the apex (24). Other studies which evaluate time of canal preparation, canal straightening after preparation and defects of these instruments during instrumentation seems valuable and are suggested.

CONCLUSION
In conclusion, though all experimental groups demonstrated greater success for Mtwo rotary instruments in removing debris, in terms of smear layer no significant differences were found.